Those familiar with structural metal goods have long recognized that cracks, pits, seams, and other defects along the surface of such goods can propagate and result in catastrophic failure. The likelihood of defect propagation is increased if the goods are repeatedly subject to high fluid pressures and/or corrosive fluids. Those familiar with oilfield tubes, such as drill pipe and casing used in hydrocarbon recovery operations, have recognized the importance of detecting such defects. Standards such as those set by the American Petroleum Institute are accordingly used to regulate the use of oilfield tubes with surface defects. Typical API standards dictate, for example, that a tubular cannot be used in a wellbore if it contains a defect which penetrates more than 121/2% of the nominal wall thickness, and that a surface defect does not warrant rejecting the tube if it penetrates less than 5% of the tubular nominal wall thickness. As may be expected, many surface defects fall within the 5% to 121/2% range, and regulations expectedly require that, if a tube with such a defect is to be used, the defect must be effectively removed to prevent propagation and thus risk failure of the tubular string in the well bore. An entire industry has thus developed to check oilfield tubes for defects.
Various techniques have been developed to detect the presence and size of defects in tubes. Although different companies tout the alleged advantages of the different techniques, the ultrasonic detection technique is widely recognized as a preferred defect detection technique due to its speed, reliability, and relatively low cost. Ultrasonic detection equipment thus may be transported to a pipe yard or similar temporary storage area where new tubulars may be inspected prior to use or reuse. If a defect which is less than the amount which mandates rejection of the tube yet greater than the allowable limit for immediate use is detected in the outer wall of the tube, the tubular may be marked and set aside. Portable grinding equipment may then be used by trained operators to grind the outer tubular surface in the vicinity of the defect, thereby effectively removing the defect and allowing the tube to be rechecked and, if found satisfactory, used in oilfield operations.
Much of the attractiveness of this tube repair procedure is lost, however, if the defect detection equipment locates a defect in the inner wall of the tube. The difficulty involved with reliably repairing such defects has caused many drilling operators to forgo repair operations, and thus hundreds of thousands of dollars are spent purchasing new tubes rather than repairing tubes with interior wall defects. While methods and equipment have been used to repair such interior rube defects, these operations to date are time-consuming and have been considered unreliable. Thus the vast majority of work performed in the tubular detection and removal industry is involved only in exterior defects.
One technique for repairing internal surface tue defects essentially involves securing a power grinder at the end of a long stick. Tubes are typically checked with detection equipment located on top of the tubes and, when an interior surface defect is located, the pipe is marked and thereafter rotated so that the defect is on the bottom of the tube. The drill motor is then inserted into the tube using the stick. Gravity bends the stick so that the motor and grinding unit rest on the bottom of the tue, and the operator activates the motor then moves the motor axially with respect to the tube by pushing and pulling the stick further in and out of the pipe in an attempt to grind off the defect. This procedure and variations of this procedure, while crude, time-consuming, and understandably not highly reliable, nevertheless is sometimes used to remove interior defects in oilfield tubes.
Another system for removing interior tube defects utilizes a special mandrel adjacent one end of this equipment for each diameter tube to be repaired. An air driven grinding wheel is pivotably mounted to the mandrel, and a control cable extends from the mandrel to a manual pressure control mechanism. An elongate rod is structurally connected at one end to the mandrel, and during use of the equipment extends from the tubular with the control cable. The operator manually reciprocates the rod and thus the mandrel and grinding motor with respect to the tube, during which time radial grinding pressure on the pivotable motor is regulated with the cable. This procedure, while somewhat more sophisticated than the earlier described technique, is still manually controlled. While the defect repair may be monitored during the grinding operation utilizing portable defect detection equipment, it is difficult for the operator to keep the grinding wheel properly on or about the defect, and this technique is physically taxing on the operator. The disadvantages of the prior art overcome by the present invention, and improved methods and apparatus are hereinafter disclosed for inexpensively and reliably moving defects from the interior wall of an elongate metal good, such as an oilfield tube. The equipment of the present invention is reliable, relatively inexpensive, not labor intensive, and may be reliably practiced at pipe inspection yards to repair oilfield tubes with interior wall defects.